High temperature weldable alloys



United States Patent 3,003,868 HIGH TEMPERATURE WELDABLE ALLOYS Robert S. Zeno and Robert F. Gill, Schenectady, N.Y-, assignors to General Electric Company, .a corporation of New York No Drawing. Filed Sept. 30, 1959, Ser. No. 843,345 4 Claims. (Cl. 75-126) This invention relates to high temperature, high pressure alloys which are useful at temperatures of the order of 1000" F. to 1100" F. and to procedures for improving the characteristics of such alloys. More particularly the invention relates to such weldable 'ferritic alloys which are characterized by desirable room temperature characteristics as well as desirable high temperature tensile, yield and creep strengths, such materials being readily fabricated into structural elements such as piping which are resistant to steam corrosion.

While present alloys for high temperature high pressure service such as in steam turbines of the order of 1050 F. and 2400 p.s.i. may be fabricated into piping and the like, generally such materials are of relatively low strength and hence the wall thickness must be so great that the only practical method of fabricating such piping is by forging and boring. It would be very desirable to so increase the strength of high temperature =weldable ferritic steels such as those of the chromium-molybdenum-vanadium class that the Wall thickness of piping rnade therefrom could be reduced to the point where piercing and drawing fabrication techniques could be ,hsed if desired resulting in only about one-third of the cost required for forging and boring. The alloys are also adaptable to casting and forging techniques for piping as well as other structural members.

A principal object of this invention is to provide new and useful weldable alloys of the chromium-molybdenumvanadium class which are characterized by improved high temperature, high pressure characteristics and to methods for improving the high temperature characteristics of such alloys.

Briefly stated the invention comprises alloys for high temperature, high pressure service which can readily be fabricated into such elements as piping by simple piercing and drawing methods which are characterized by improved high temperature, high pressure characteristics and which are readily heat treated to develope the maximum qualities for high temperature service. The alloys are also readily forged and cast.

The alloys of the invention have the following percent by weight composition:

Specific Examples, Percent Constituent Range, Percent 0. -10-0. '20 0. 16 0. 20 0.1 :16 0. 75-3. 0 1.12 1. 04 1.02 1. 41 0. 75-1. 75 0. 97 0. 96 1.00 0. 96 0. 30-1. 0 0. 54 0.81 0. 42 0. 56 0. 30-0. 80 0. 80 0. 62 0. 33 0. 39 0. 30-0. 50 0.30 0. 28 0.29 0. 33 Remainder Rem. Rem. Rem. Rem.

The desirable high temperature characteristics of the present alloys may be developed by heat treating to normalize the material. The alloys can be normalized for at least four hours at temperatures of from 1800 F. to 2050 F., air cooled and then tempered to strength at temperatures varying from 1150 F. to 1400 F. for from to 30 hours.

Those features of the invention which are believed to be patentable are specifically set forth in the claims appended hereto. The invention will, however, be better .F. to 2050 2 understood and further objects and advantages thereof appreciated from a consideration of the following description.

The alloys of the present invention represent a'balance of ingredients or constituents which combine to provide the improved characteristics which appertain thereto.

The carbon content of the alloy should be held to from 0.10 to 0.20 if the optimum combination of high temperature strength and weldability is to be obtained. With carbon contents less or greater than the above, it has been found that the high temperature rupture strength of the steel is significantly lowered. Less than about 0.10% carbon produces .an alloy which .does not have suflicient hardenabilityand does not have enough carbide for adequate strength. Also, more than about 0.20% carbon produces an alloy which does not weld readily. The chromium content of the alloy is likewise quite critical and should be held between about 0.75% and 3.0%. It has been found that less than about 0.75% chromium results in an alloy having inadequate ,orridation and graphitization resistance. More than.abou t 3% chromium on the whole results in an undesirable reduction in creep and rupture strengths. The molybdenum content of the present alloy should be maintained between about 0.75% to 1.75%. Less than about 0.75 molybdenum results in an alloy having an undesirably ,loW creep and rupture strength. More than about 1.75% molybdenum results in the devel pment of molybdenum carbides at the expense of vanadium carbides which again gives a lower creep and rupture strength. In has been found that increasing the vanadium content to about .3 o .0% and pr fe y t from a out 1 -50% to 0. 5 f l owed y h p c a a t tment p cri ed, substantially increases the creep strength and rupture strength at elevated temperature. Manganese should be present in the present alloys in sufiicient amount of form manganese sulfide and to favorably influence the hardenability. Amounts of manganese' from about -0. 3%',to 0.80% have been found to well suit this purpose in the Present instance. Lower amounts of manganese resultin insuflicient manganese sulfide formation and more than about 0.80% manganese results in lowered rupture strength. The silicon in the present alloys is present in amounts of from about 0.30 to 0.50% which have been found to be sufiicien-t for the deoxidization purposes which the silicon serves. Lower amounts of silicon lresult in in u i i t e i a on- 0n th ther h d higher amo o sil n r u t in .a educe toug ne s a d a poor weldability. As pointed out above, the alloys of the present invention can be normalized in order to produce the substantially bainitic structure which is desirable for optimum rupture strength. For example the alloys may be normalized for at least four hours at from about 1800 F., air cooled and then tempered to strength from about 1150 F. to 1.400" F. for periods ranging from about 10 to 30 hours. Alternatively the alloys may be annealed for about four hours at temperature of from about 1000 C. to 1100 C. and then furnace cooled.

When a prior art material containing about 1% chromium and 1% molybdenum and about 0.25% vanadium was normalized at 1950 F. for about four hours and tempered at 1300 F. for about twelve hours, the maximum 100,000 hour rupture strength at 1050 F. was 18,500 p.s.i. and at 1100 F. was 10,000 p.s.i. On the other hand when the present material as set forth in Example 1 above was so treated, the 100,000 hour rupture strength at 1050 F. had increased to 22,000.p.s.i. and the 1100 F. rupture strength had increased about 50% to 15,000 psi. When the material of Example 2 containing about 1.04% chromium, 0.96% molybdenum, 0.81% vanadium and 0.20% carbon was normalized as 100,000 Hour Rupture Normalize Temper. Strength (p.s.1.)

1,000 F. 1,050 F. 1,100 F.

From the above it will be noted that the best alloys are produced at normalizing temperatures of about 1950 F. and tempering temperatures of about 1325 F.

Not only are the rupture strengths of the present materials highly desirable but their other high temperature characteristics are very useful. For example, a material such as that of Example 1 when annealed as described above has at 1150 F. a tensile strength of 35,900 p.s.i. and 0.02% yield strength of 27,150 psi, a proportional limit of 23,600 p.s.i., a percentage of elongation in 2 inches of 47% and a reduction in area of 85%. When the same material is normalized as described above, the 1150 F. tensile strength is 55,300 p.s.i. the 0.02% yield strength is 41,500 p.s.i., the proportional limit is 35,600 psi, the elongation in 2 inches is 32% and the reduction in area is 86%.

There are provided by the present invention alloys which are characterized by improved physical characteristics at elevated temperatures combined with good Weldability. These characteristics are so improved that they may readily be fabricated into structures for steam turbines and the like which have a substantially thinner cross section than was heretofore required. Such improved high temperature strength characteristics make possible the fabrication of pipe from such alloys by piercing and drawing procedures which are much more economical than the forging and boring methods required with former lower strength alloys, such forging and boring being necessitated by the fact that the piping made therefiorn required a greater wall thickness to meet strength requirements.

What we claim as new and desire to secure by Letters Patent of the United States is:

1. An alloy for high temperature service comprising by weight from about 0.10 to 0.20% carbon, 0.75 to 3.0% chromium, 0.75 to 1.75% molybdenum, 0.30 to 1.0% vanadium, 0.30 to 0.80% manganese and 0.28 to 0.50% silicon with the remainder essentially iron said alloy being normalized for about four hours at from about 1800 F. to 2050 F., air cooled and then tempered at a temperature of from about 1150 F. to 1400 F. for from to hours to strength, said alloy having a bainitic structure after heat treatment.

.finery Service up to 1100 2. An alloy for high temperature service comprising by weight from about 0.20% carbon, 1.04% chromium, 0.96% molybdenum, 0.81%vanadium, 0.62% manganese and 0.28% silicon with the remainder essentially iron said alloy being normalized for about 4 hours at from about 1800 F. to 2050" E, air-cooled and then tempered at a temperature of from about 1150 F. to 1400 F. for from 10 to 30 hours to strength, said alloy having a bainitic structure after heat treatment.

3. An alloy for high temperature service comprising by weight from about 0.1% carbon, 1.02% chromium, 1.00% molybdenum, 0.42% vanadium, 0.33% manganese and 0.29% silicon with the remainder essentially iron said alloy being normalized for about 4 hours at from about 1800 F. to 2050 F., air-cooled and then tempered at a temperature of from about 1150 F. to 1400 F. for from 10 to 30 hours to strength, said alloy having a bainitic structure after heat treatment.

4. An alloy for high temperature service comprising ,by, weight from about 0.16% carbon, 1.41% chromium,

0.96% molybdenum, 0.56% vanadium, 0.39% manganese and 0.33% silicon with the remainder essentially iron said alloy being normalized for about 4 hours at from about 1800 F. to 2050 E, air-cooled and then tempered at a temperature of from about 1150" F. to 1400 F. for from 10 to 30 hours to strength, said alloy having a bainitic structure after heat treatment.

References Cited in the tile of this patent UNITED STATES PATENTS OTHER REFERENCES Gemmill et al.: Engineering, vol. 180, Dec. 16, 1955,

pages 824-827. Published by Engineering, Ltd., London, England.

Rankin et al.: Transactions ASME, vol. 73, 1951, pages 891-904. Published by the American Society of Mechanical Engineers, New York, NY.

Smith: Chromium-Molybdenum and Chromium- Molybdenum Vanadium Steels for Power Plant and Re- F.: An appraisal of the literature. A paper presented at the ASME Annual Meeting, Nov, 25-30, 1956 and reprinted by Climax Molybdenum Co., New York, N.Y. 

1. AN ALLOY FOR HIGH TEMPERATURE SERVICE COMPRISING BY WEIGHT FROM ABOUT 0.10 TO 0.20% CARBON, 0.75 TO 0.0% CHROMIUM, 0.75 TO 1.75% MOLYBDENUM, 0.30 TO 1.0% VANADIUM, 0.30 TO 0.80% MANGANESE AND 0.28 TO 0.50% SILICON WITH THE REMAINDER ESSENTIALLY IRON SAID ALLOY BEING NORMALIZED FOR ABOUT FOUR HOURS AT FROM ABOUT 1800*F. TO 2050*F., AIR COOLED AN THEN TEMPERED AT A TEMPERATURE OF FROM ABOUT 1150*F. 1400*F. FOR FROM 10 TO 30 HOURS TO STRENGTH, SAID ALLOY HAVING A BAINITIC STRUCTURE AFTER HEAT TREATMENT. 